The present invention relates to resistor networks and more particularly to R-2R resistor networks employed in particular with D/A (digital-to-analog) converters.
Prior art R-2R resistor networks are described in "Technische Informationen fur die Industrie" No. 791221 "Digital/Analog-Wandler Grundlagen und Anwendungen" by Valvo (December 1979) and to the German technical journal "Elektronikpraxis" No. 8 (August 1979) pp. 3 to 16.
When such R-2R resistor networks are employed with D/A converters, there are used electronic switches with the aid of which each of the 2R resistors, in accordance with a digital word, can be applied either to ground or to another reference point. The reference point is determined by the type of the D/A converter with a distinction being made between current-controlled and voltage-controlled D/A converters.
When manufacturing the electronic switches, especially in the case of a monolithic integration thereof within a solid state circuit, the switch resistances thereof are subject to variations owing to process parameter fluctuations during production. In the case of the R-2R resistor networks, this has unpleasant effects upon the accuracy.
When considering, for example, the R-2R resistor network of an 8-bit D/A converter having an accuracy of 1/2LSB(least significant bit), high demands have to be placed on the series resistors R and the shunt 2R resistors, as well as on the accuracy of the switch resistors. In the case of an absolute switch resistance variation of .+-.20% and an ideal R-2R resistor network, the RS/2R ratio may in the utmost be 0.004. In the case of a 2R resistor of 8 kilohms there is obtained a switch resistance of 32 ohms. A switch having such a low resistance can only be realized with the aid of large-surface dimensioning. Owing to the high input capacitances, there are obtained disadvantageously long switching times.